Problem: The scale on a map is 3cm : 3km. If the distance between two cities is 27km, how far apart in cm are the two cities on the map?
The scale means that every 3cm on the map corresponds to an actual distance of 3km. An actual distance of 27km is the same as 9 $\cdot$ 3km. The distance between the two cities on the map is 9 $\cdot$ 3cm, or 27cm.